1. Field of the Invention
The present invention relates to a piezoelectric transformer, and particularly to a piezoelectric transformer which is preferably used, for example, for driving a cold cathode fluorescent lamp (hereinafter referred to as CFL) which is used as a backlight for a liquid crystal display (hereinafter referred to as LCD) panel.
2. Description of the Related Art
Reductions in size and power consumption are required of portable devices equipped with LCD panels such as notebook type personal computers. A CFL which is widely used as a backlight for an LCD panel requires a high voltage of not less than 1 kV to initiate lighting and a high voltage of several hundred volts while it is continuously lit. To supply such high voltages, high voltage step-up wound transformers have been used, but they are reaching their performance limit in terms of efficiency, size and the like.
Recently, a CFL lighting unit using a piezoelectric transformer of smaller size and higher efficiency has been developed for an LCD panel (see Nikkei Electronics, 1994, Nov. 7 (No. 621), pp. 147-157). The piezoelectric transformer used in the unit has a structure called ROSEN type and still has an insufficient voltage step-up ratio, and hence the CFL lighting unit which has been put into practice uses a wound transformer in a stage preceding the piezoelectric transformer.
FIGS. 1A to 1D are diagrams for explaining a conventional ROSEN type piezoelectric transformer element. FIG. 1A is a perspective view, FIG. 1B is a cross-sectional view, FIG. 1C is a graph showing stress distribution, and FIG. 1D is a graph showing amplitude distribution.
A primary-side electrode 22 is disposed on the top face 12 of a rectangular parallelepipedic piezoelectric ceramics substrate 10 to cover the left (primary) half thereof, and also a primary-side electrode 24 is disposed on the bottom face 14 of the piezoelectric ceramics substrate 10 opposite to the primary-side electrode 22. The portion of the piezoelectric ceramics substrate 10 between the primary-side electrode 22 and the primary-side electrode 24 is polarized in the thicknesswise direction between the top face 12 and the bottom face 14. A secondary-side electrode 72 is disposed on a secondary-side end surface 17 perpendicular to both the top face 12 and the bottom face 14, and the portion of the piezoelectric ceramics substrate 10 between the secondary-side electrode 72 and the primary-side electrodes 22 and 24 is longitudinally polarized. One end of a power source 200 is connected to the primary-side electrode 22 via a connecting portion 122, and the other end of the power source 200 is connected to the primary-side electrode 24 via a connecting portion 124. The secondary-side electrode 72 is connected to a CFL 400 used as a load at one end thereof, and the other end of the CFL 400 is connected to the primary-side electrode 24 via a connecting portion 124.
When a voltage from the power source 200 is applied between the primary-side electrodes 22 and 24, an electric field is applied to the left half of the piezoelectric ceramics substrate 10 in the thicknesswise direction thereof. This excites longitudinal vibration of the longitudinal direction due to a piezoelectric transverse effect involving a displacement perpendicular to the direction of polarization, whereby a piezoelectric transformer element 100 vibrates as a whole. Further, a mechanical distortion occurs within the right half of the piezoelectric ceramics substrate 10 in the longitudinal direction. This causes a voltage having the same frequency as that of the primary voltage applied between the primary-side electrodes 22 and 24 to be output from the secondary-side electrode 72 due to a piezoelectric longitudinal effect involving the generation of a potential difference in the direction of polarization. By applying a driving voltage having the same frequency as the resonant frequency of the piezoelectric transformer element 100 between the primary-side electrodes 22 and 24, a very high voltage step-up ratio can be obtained.
However, in order to use a piezoelectric transformer using lead zirconate-titanate (PZT) ceramics as a step-up transformer for a CFL lighting inverter used in the LCD panel of an A4-sized notebook type personal computer, a considerably high input voltage must be applied. Accordingly, an obtained voltage step-up ratio has still been insufficient.